Field of the Disclosure
The present disclosure relates to a liquid crystal display (LCD) device using a direct type backlight unit.
Discussion of the Related Art
As a backlight unit applied to a related art LCD device, an edge-light type backlight unit has been known in which a light source is disposed at an edge of a light guide plate and light emitted from the light source is provided to a liquid crystal panel.
However, since the edge-light type backlight unit requires the light guide plate, there is a problem that, particularly, a large-sized LCD device becomes heavier. Additionally, in the edge-light type backlight unit, since the light source is disposed at the edge of the light guide plate, a bezel area around the liquid crystal panel needs to be widened, and there is a problem that the design is not good.
Meanwhile, in comparison with the edge-light type backlight unit, a direct type backlight unit has been known in which a light source is disposed at a rear side of a liquid crystal panel and light emitted from the light source is diffused and concentrated by combinations of various optical films (a diffusion sheet, a prism sheet, a lens sheet, etc.) and is provided to the liquid crystal panel.
In addition, as the direct type backlight unit, a backlight assembly including a plurality of light sources and a plurality of optical lenses corresponding to the light sources has been suggested in which each optical lens includes a central lens portion having a convex shape, a peripheral lens portion having a concave shape and formed outside the central portion, and a light guiding portion extending from the peripheral portion (referring to Japanese Patent No. 4959971, for example).
However, since the direct type backlight unit is disposed at the rear side of the liquid crystal panel, there is a problem that a thickness of a display device becomes thicker. Moreover, as a common problem of the LCD device, there is a problem that utilization efficiency of light is low because most of the light emitted from the light sources is absorbed by a polarizer, which is disposed at a light incident surface of the liquid crystal panel (a surface adjacent to the backlight unit).
A method of using a lens material having a high refractive index has been suggested to decrease a thickness of the direct type backlight unit. However, in general, the high refractive index is about 1.7, and to further improve the thickness, it is needed to use a special material, which is expensive.
Furthermore, to increase the utilization efficiency of light of the LCD device, another method has been known in which a polarizing optical film is disposed between the liquid crystal panel and the direct type backlight unit, P wave of light from the directly type backlight unit (polarized component wave parallel to the light incident surface) is transmitted while S wave (polarized component wave perpendicular to the light incident surface) is reflected toward the direct type backlight unit, and the S wave is reflected by a reflector disposed around the light sources of the direct type backlight unit and is recycled. An example of the polarizing optical film is a dual brightness enhancement film (DBEF), which includes multiple layers having different refractive indexes.
However, in the LCD device having the above-mentioned structure, the size of the polarizing optical film should be almost the same as the size of the liquid crystal panel. Moreover, since the polarizing optical film includes the multiple layers, there is a problem that the costs increase particularly when the LCD device has a large size. Furthermore, conversion efficiency from S wave to P wave of the polarizing optical film is not 100%, and a new breakthrough is needed to accomplish further high utilization efficiency of light at low cost.
In general, a cold cathode fluorescent lamp (CCFL) and a light emitting diode (LED) have been known as the light sources of the direct type backlight unit. The LED is suitable for the light sources of the LCD device because the LED has low power consumption and a small size as compared to the light sources.
Meanwhile, it is important to increase color reproduction characteristics of the LCD device according to practical applications of an organic EL display. Color display of the LCD device can be realized by making light with some wavelength range of light emitted from a white light source absorbed by a color filter. To increase the color reproduction characteristics of the LCD device, it is effective to narrow the wavelength range of light passing through the color filter, and on the other hand, there is a problem of lowering utilization efficiency of light. To solve the problem, it is needed that light with a narrow wavelength range corresponding to the three primary colors of light is emitted from the light source and the wavelength range of light emitted from the light source is matched with the wavelength range of light transmitted by the color filter.
With this background, recently, a backlight unit including a combination of the LED and a quantum dot (QD) has been actively studied (referring to Japanese Patent Publication No. 2013-539170, for example).
The quantum dot uses light from the LED as an excitation source and emits light with a longer wavelength than that of the excitation source. In addition, the wavelength of the light emitted by the quantum dot can be controlled by changing the type and the size of the quantum dot. Moreover, the quantum dot has high quantum efficiency approaching that of YAG (yttrium aluminum garnet) fluorescent material. Therefore, it is possible to obtain a backlight unit having desired emission color, high brightness and emission spectrum with a narrow full width at half maximum (FWHM) by using the quantum dot. Furthermore, it is possible to manufacture an LCD device having wide color gamut due to the backlight unit using the quantum dot.
However, there is a problem that the quantum dot is degraded in a short time when it receives light from the LED in the presence of moisture and oxygen. Additionally, when the quantum dot is close to the LED of the excitation source, the degradation of the quantum dot is accelerated in circumstances of large amount of light and high temperature.